Cable Television
A significant proportion of television entertainment is presently delivered by way of cable television systems. In general, these systems can receive broadcast television signals on a common antenna or set of antennas, amplify the signals, and distribute them along transmission lines either at their original frequencies or after conversion to different frequencies. The transmission lines are connected to the television sets of the system's subscribers, usually through a decoding or filtering device which is intended to prevent non-subscribers from receiving the signals. Frequency conversion is often necessary in systems which handle programs broadcast from outside the local area, since there is a good chance that signals will be received from two different stations which operate on the same channel. (A standard broadcast television channel in the United States is a band of frequencies 6 MHz in width.)
Cable television systems are also well suited to transmit nonbroadcast programming to their subscribers. Such programs originate within the cable television system itself, for example by displaying a motion picture to a television camera and using the resulting electrical signal to modulate a carrier freqency.
In either case, cable television systems consist of four main parts: a head end, the main trunk cable distribution system, feeder cables and drops from feeder cables to the subscriber's TV set. A cable television head end includes the television receiving antennas for off-air pick up and microwave and/or satellite receivers for other direct television programming as well as equipment that originates local programming. From the head end, trunk lines transport the signal to feeder lines which carry signals past each home.
The integrity of trunk cables is not meant to be breached by tapping off signals for direct distribution to subscriber homes. Rather this is accomplished with a feeder cable. The signals on feeder cables come from amplifiers (bridger amplifiers) bridged across the trunk cable at distribution points. Therefore, in many cases, feeder cables must parallel or backtrack side-by-side with the trunk cable in order to have a cable that can be tapped to feed a signal to subscribers. Signal losses in feeder cables may require line extender amplifiers approximately every 1500 feet. These multi-channel amplifiers cause problems themselves. Repeated amplification of broadband television signals causes noise, inter-modulation distortion and echoes, increasing with each amplification, thereby limiting the distance that signals can be transmitted while maintaining a given standard of picture quality.
Cable losses increase with frequency as well as with distance. Thus, the superband VHF portion of the cable spectrum, 216 to 300 MHz, is subject to greater loss than the highband and lowband VHF, 54 MHz to 216 MHz.
While the TV channel allocation spans the spectrum from 54 to 890 MHz, a cable television system utilizes only a small part of this spectrum. The first cable television systems were built in the early 1950's to provide broadcast channels to subscribers in areas that could not receive off-air signals. These early 5-channel systems used the standard television frequencies from 54 to 88 MHz for distribution of these broadcast channels. When the state of the art advanced to include 12 channels, cable used the high VHF spectrum of 174 to 216 MHz as well. These 12 channels could be received without a converter. To increase to 21 channels, cable systems used the midband spectrum from 108 to 174 MHz to add nine additional channels; these channels required using a frequency converter since the television set tuners could not accommodate these midband channels. Current 35-channel systems add 14 additional channels by using 216 to 300 MHz. To accomplish this, frequency converters were upgraded. Plans exist now to add another 25 channels, extend the cable television spectrum to 450 MHz. Thus, as cable television systems add channels, they use increasingly higher frequencies. This can only be done at the cost of greater signal attenuation.
The upper frequency limit on trunks is established by the performance of linear broadband amplifiers and by trunk cable attenuation, which increases with increasing frequency. Feeder cable length is limited to about 1500 feet by the attenuation at the highest frequencies carried on the feeder. Therefore, the gap between the highest VHF television frequency at 216 MHz and the lowest UHF television frequency at 470 MHz establishes a natural barrier to attempted carriage of both VHF and UHF signals on a cable system.